1. Field of the Invention
The present invention relates to a linear actuator.
2. Description of the Related Art
FIG. 3 is a schematic cross sectional view of a conventional linear actuator. The linear actuator of FIG. 3 comprises a stator assembly 10, a rotor assembly 20, a rear end cap 30, an output shaft 40, and a front end cap 50.
The stator assembly 10 is composed of two stator units, one of which is structured such that two stator yokes 13a, 13b shaped into a ring oppose each other so as to sandwich therebetween a bobbin 12 having a winding 11 provided therearound, and the other of which is structured such that two stator yokes 16a, 16b shaped into a ring oppose each other so as to sandwich therebetween a bobbin 15 having a winding 14 provided therearound. The two stator units structured as above are coaxially stacked on each other forming a hollow-cylinder looking like a doughnut. The stator yokes 13a, 13b each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. In the same way, the stator yokes 16a, 16b each have an array of pole teeth and are coupled to each other with their respective pole teeth intermeshing with each other. The pole teeth constitute the inner circumference of the stator assembly 10. The windings 11, 14 are responsible for exciting the pole teeth. The stator yokes 13a, 13b, and 16a, 16b, and the bobbins 12, 15 are integrally fixed together by resin injection-molding.
The rotor assembly 20 is housed in the stator assembly 10. The rotor assembly 20 is composed of a rotor magnet 21, and a resin segment 22, and is shaped into a hollow-cylinder. The rotor magnet 21 has a plurality of magnetic poles, and constitutes the outer circumference of the rotor assembly 20 thus opposing the pole teeth of the stator assembly 10 with a predetermined gap therebetween. The resin segment 22 is shaped tube-like, and disposed inside the rotor magnet 21, and a female screw 23 is fixedly attached inside the resin segment 22.
The rear end cap 30 is disposed at the rear end face of the stator assembly 10 so as to cover the hollow of the stator assembly 10. The rear end cap 30 has a cavity 31 at its inner side facing the rotor assembly 20 and has a rear ball bearing 32 fitted into a circular recess formed coaxially with the cavity 31. The ball bearing 32 supports rotatably the rear end portion of the rotor assembly 20.
The output shaft 40 is shaped round in its cross section, has a male screw 41 formed at its rear end portion, has a stopper pin 24 disposed at its frontward portion, and has its rearward portion inserted through the rotor assembly 20. The male screw 41 engages threadedly with the female screw 23 of the rotor assembly 20, whereby the output shaft 40 travels in the axial direction linearly without turning or with less than one turn when the rotor assembly 20 rotates. In this connection, the stopper pin 24 prohibits or restricts rotation of the output shaft 40 within one turn.
The front end cap 50 is attached to the front end of the stator assembly 10 so as to cover the hollow of the stator assembly 10 housing the rotor assembly 20. The front end cap 50 has a round center hole 51, and the output shaft 40 is inserted through the center hole 51 so as to have its front end portion sticking out from the front end cap 50. The front end cap 50 has a circular recess 52 coaxial with the center hole 51, and has a groove 53 extending parallel to the length of the output shaft 40. The recess 52 receives a front ball bearing 54 fitted thereinto, which rotatably supports the front end portion of the rotor assembly 20. The aforementioned stopper pin 24 is lodged in and guided by the groove 53 so as to prohibit or restrict rotation of the output shaft 40, and to restrict the frontward travel amount of the output shaft 40, and, in some cases, the rearward travel amount thereof as well.
In the above described linear actuator of FIG. 3, when current is caused to flow in the windings 11, 14, the pole teeth of the stator assembly 10 are excited thereby rotating the rotor assembly 20 due to magnetism of the rotor magnet 21 having magnetic poles. When the rotor assembly 20 rotates, the rotational movement of the rotor assembly 20 is converted into linear movement of the output shaft 40 by means of the female screw 23 of the rotor assembly 20 threadedly engaging with the male screw 41 of the output shaft 40. The output shaft 40 travels in a reciprocating manner within the length of the groove 53 in response to the rotational direction of the rotor assembly 20. The output shaft 40 stops and reverses its movement when the rotor assembly 20 stops and reverses its rotation.
The linear actuator above described may encounter troubles incurred when the output shaft 40 stops its movement, that is, when the rotor assembly 20 is caused to stop its rotation. The male screw 41 of the output shaft 40 has a predetermined length defined by its proximal end portion 41a, and in some linear actuators in which the rearward travel amount of the output shaft 40 is restricted by the proximal end portion 41a, the rotor assembly 20 is caused to stop its rotation when the proximal end portions 41a of the male screw 41 touches the female screw 23. In this case, the thread of the female screw 23 may possibly bite into the proximal end portion 41a depending on the magnitude of inertial force of the rotation of the rotor assembly 20 at the time of touching, which, depending on the degree of the biting, can cause a critical problem that the rotor assembly 20 will not start off its rotation in the reversed direction therefore failing to move the output shaft 40. On the other hand, if the length of the male screw 41 is increased to keep off the screw biting problem, the output shaft 40 is caused to stop its rearward movement when the stopper pin 24 touches the front end of the rotor assembly 20 or the inner ring of the front ball bearing 54. In this case, since the touching area of the stopper pin 24 therewith is positioned outside the pitch diameter of the female and male screws 23, 41, an extra torque is required for the rotor assembly 20 to duly start its rotation in the reversed direction thus, in the worst case, making it possibly happen that the rotor assembly 20 will not start off its rotation.
A linear actuator to address the above problems is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H06-335228 and will be explained below based on reference numbers in FIG. 3. The linear actuator disclosed therein has a pointed stopper disposed at the center of the cavity 31 of the rear end cap 30. In the linear actuator, the output shaft 40 is caused to stop its rearward movement when the rear end surface of the output shaft 40 touches the pointed stopper. This structure eliminates the two problems described above, specifically one is that the thread of the female screw 23 bites into the proximal end portion 41a of the male screw 41, and the other is that an increased torque is required for the rotor assembly 20 to duly start off its rotation in the reversed direction. However, since the stopper has a pointed head, the head of the stopper can possibly be readily worn away or damaged due to the rear end surface of the output shaft 40 repeatedly touching the head. If the head of the stopper is worn away or damaged, the output shaft 40 cannot be stopped precisely at a place originally determined thus failing to perform an accurate control.